The present invention relates to a pressure equalisation device adapted to be operatively connected to an enclosure to ensure that, during operation, the pressure in the enclosure is substantially equal to atmospheric pressure at all times.
In many situations, sensitive equipment, such as electrical equipment, printed circuits, pneumatic control circuits and hydraulic control circuits, is used in exposed environmental conditions, from which the equipment must be protected. In many cases, the problem is solved by encasing the equipment entirely in a protective enclosure which is completely sealed from the exterior atmosphere. However, such a solution is not possible where the equipment to be protected includes pressure members or transducers which are required to be responsive to atmospheric pressure. Atmospheric pressure, of course, varies with atmospheric conditions and this problem is compounded when the equipment is used on a vehicle whose height above sea level can vary throughout each journey. In such installations, it is necessary to have the interior of a protective enclosure vented to atmosphere through an air vent or pressure equalisation device to ensure that the air pressure within the protective enclosure is maintained substantially at atmosphere.
Particularly, when mounted on vehicles such as rail cars, the enclosure and the air vents are subjected to a harsh environment which includes dust and water and it is necessary to exclude such foreign matter from entering the enclosure through the pressure equalisation device, to prevent damage to the equipment inside the enclosure.
The present invention seeks to provide a solution to this problem and, particularly but not essentially, to provide a pressure equalisation device for an enclosure which can meet the requirements of BS EN 60529: 1992 IP 66. IP 66 is an industry standard for instrument protection, the first digit indicating a level of protection against solid objects, 6 being no ingress of dust possible, the second digit indicating a level of protection against liquids, 6 being protection against powerful jets of water.
According to the present invention there is provided a pressure equalisation device adapted to be operatively connected to an enclosure, the device including an air passage which, when the device is so connected to the enclosure, provides a fluid communication between the interior of the enclosure and atmosphere, the air passage containing a barrier wall comprising an air permeable membrane forming a particulate filter.
Preferably, the barrier wall extends across the air passage and a predetermined distance along the passage so as to have an area subjected to the air flow which is greater than the cross-sectional area of the passage.
Preferably, the device comprises a carrier for the barrier wall, the carrier being sealingly insertable in a bore in a wall of the enclosure, the air passage being formed in part by said bore.
The permeable membrane preferably forms a micro particulate filter, which may be formed of a polymer material.
Preferably, the barrier wall also forms a water barrier preventing the increase of water into the enclosure. The device may also include a baffle means to inhibit water entering the air passage. When the device comprises a carrier for the barrier wall, the baffle may serve to clamp the carrier to the enclosure.
The device preferably includes a second air passage in parallel with the first air passage for venting the enclosure, the second air passage including a one-way vent valve adapted to permit air flow out of the enclosure through said second air passage but to prevent air flowing into the enclosure through said second air passage. Preferably, the second air passage is formed in said carrier, the one-way vent valve comprising a resilient diaphragm biased to close said second air passage and being urged to a closed position by air flowing in a direction towards the enclosure and being opened against the resilient bias by air flowing out of the enclosure.